A pixel circuit including a light emitting element, a driving transistor, with a drain terminal thereof connected to a cathode terminal of the light emitting element, that applies a drive current to the light emitting element, a capacitor element connected to a gate terminal of the driving transistor, and a switching transistor connected between a first terminal of the capacitor element on the side of the gate terminal and a data line through which a desired program signal flows, in which the driving transistor is an inorganic oxide thin film transistor whose OFF-operation threshold voltage is a negative voltage, and a source terminal of the driving transistor and a second terminal of the capacitor element are connected to a common power source that supplies a predetermined common voltage.
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5. A pixel circuit, comprising:
a light emitting element; a driving transistor, with a source terminal thereof connected to an anode terminal of the light emitting element, that applies a drive current to the light emitting element; a capacitor element connected to a gate terminal of the driving transistor; and a switching transistor connected between a first terminal of the capacitor element on the side of the gate terminal and a data line through which a desired program signal flows;
an active matrix substrate in which a great number of the pixel circuits, in which the drive transistors are inorganic oxide thin film transistors having a negative voltage as a threshold voltage for an OFF operation are arranged;
a data driving circuit for supplying the program signal; and
a common power source for supplying a predetermined common voltage to a cathode terminal of the light emitting element and a second terminal of the capacitor element; wherein
the voltage value vprg of the program signal satisfies the condition: Vprg≧0;
the common power source supplies the common voltage when the switching transistor is turned ON and the voltage value vprg of the program signal is applied to the gate terminal of the driving transistor; and
the common voltage VB and the threshold voltage VTH of the driving transistor are set to values that satisfy the condition: VB≧−VTH.
1. A display apparatus comprising:
pixel circuits, each equipped with: a light emitting element; a driving transistor, with a drain terminal thereof connected to a cathode terminal of the light emitting element, that applies a drive current to the light emitting element; a capacitor element connected to a gate terminal of the driving transistor; and a switching transistor connected between a first terminal of the capacitor element on the side of the gate terminal and a data line through which a desired program signal flows;
an active matrix substrate in which a great number of the pixel circuits, in which the drive transistors are inorganic oxide thin film transistors having a negative voltage as a threshold voltage for an OFF operation are arranged;
a data driving circuit for supplying the program signal; and
a common power source for supplying a predetermined common voltage to a source terminal of the driving transistor and a second terminal of the capacitor element; wherein
the voltage value vprg of the program signal satisfies the condition: Vprg≧0;
the common power source supply the common voltage when the switching transistor is turned ON and a voltage value vprg of the program signal is applied to the gate terminal of the driving transistor;
the common voltage VB and the threshold voltage VTH of the driving transistor are set to values that satisfy the condition: VB≧−VTH.
2. The display apparatus of
3. The display apparatus of
4. The display apparatus of
6. The pixel circuit of
7. The pixel circuit of
8. The pixel circuit of
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1. Field of the Invention
The present invention generally relates to a pixel circuit and display apparatus having a light emitting element driven by active matrix method, and more particularly to a pixel circuit using an inorganic oxide thin film transistor.
2. Description of the Related Art
Display devices using light emitting elements, such as organic EL element and the like, are proposed for use in various fields including televisions, cell phone displays, and the like.
Generally, the organic EL element is a current-driven type light emitting element, thus pixel circuits including an organic EL element proposed have a configuration like that shown in
The pixel circuit shown in
In conventional pixel circuits, low-temperature polysilicon or amorphous silicon thin film transistors are used as the switching transistor and driving transistor.
The low-temperature polysilicon thin film transistor may provide high mobility and high stability of threshold voltage, but has a problem that the mobility is not uniform. The amorphous silicon thin film transistor may provide uniform mobility, but has a problem that the mobility is low and threshold voltage varies with time. The non-uniform mobility and instable threshold voltage appear as irregularities in the display image.
Consequently, Japanese Unexamined Patent Publication No. 2003-255856 proposes a pixel circuit having therein a compensation circuit for correcting the threshold voltage.
The provision of the compensation circuit, however, causes the pixel circuit to become complicated, resulting in increased cost due to low yield rate and low aperture ratio.
As such, thin film transistors made of inorganic oxide films, as typified by IGZO, have recently been drawing attention. The thin film transistors made of inorganic oxide films allow low-temperature film forming and have features of providing sufficient mobility, highly uniform mobility, and low threshold voltage variation with time.
Where thin film transistors are fabricated with inorganic oxide films in order to obtain various desired characteristics and when trying to obtain desired current characteristics, however, the threshold voltage that causes the transistors to perform OFF operation may sometimes become a negative voltage.
For example, when trying to control a thin film transistor, used as the driving transistor whose OFF-operation threshold voltage is a negative voltage like that described, for example, “Highly Stable Ga2O3—In2O3—ZnO TFT for Active-Matrix Organic Light-Emitting Diode Display Application”, C. J. Kim et al., IEDM (International Electron Device Meeting) 2006, Samsung Advanced Institute of Technology (Non-Patent Document 1) by the data driving circuit of a conventional organic EL display device, the minimum setup value of the gate voltage of the driving transistor of the conventional data driving circuit is 0 v, so that a minimum drive current, which is the value when gate-source voltage VGS of the driving transistor is 0 v, flows through the organic EL element, thus unable to cause the EL element to stop the emission.
Use of a thin film transistor whose OFF-operation threshold voltage is a negative voltage as driving transistor 102 results in that driving transistor 102 is unable to perform OFF operation as shown in
In order to solve the problems described above, it is conceivable to provide a voltage source to set the ground wire of the pixel circuit at a voltage (VA) higher than 0 v, as shown in
It is also conceivable to set the ground wire of the data drive circuit that supplies a program signal at a voltage higher than 0 v, thereby causing the program signal to become negative. But in order to ensure the data connection level with an external device, it is necessary to newly develop a dedicated IC, which becomes a cost increase factor of the display device.
In view of the circumstances described above, it is an object of the present invention to provide a pixel circuit that uses an inorganic oxide thin film transistor whose OFF-operation threshold voltage is a negative voltage, yet does not increase power consumption and allows the use of a conventional drive circuit, and a display apparatus that uses the pixel circuit.
A first pixel circuit of the present invention is a circuit including:
a light emitting element;
a driving transistor, with a drain terminal thereof connected to a cathode terminal of the light emitting element, that applies a drive current to the light emitting element;
a capacitor element connected to a gate terminal of the driving transistor; and
a switching transistor connected between a first terminal of the capacitor element on the side of the gate terminal and a data line through which a desired program signal flows, wherein:
the driving transistor is an inorganic oxide thin film transistor whose OFF-operation threshold voltage is a negative voltage; and
a source terminal of the driving transistor and a second terminal of the capacitor element are connected to a common power source that supplies a predetermined common voltage.
A first display apparatus of the present invention is an apparatus, including:
an active matrix substrate on which the first pixel circuit of the present invention described above is disposed in a large number;
a data drive circuit that supplies the program signal; and
a common power source that supplies a predetermined voltage to the source terminal of the driving transistor and the second terminal of the capacitor element,
wherein a voltage value VB of the common voltage and a voltage value Vprg of the program signal are set such that the threshold voltage VTH, the voltage value VB of the common voltage, the voltage value Vprg of the program signal, and a desired gate-source voltage VGS to be set for the driving transistor satisfy the relationships of Formulae (1) and (2) below.
VB≧−VTH (1)
Vprg=VGS−VB (2)
A second pixel circuit of the present invention is a circuit, including:
a light emitting element;
a driving transistor, with a source terminal thereof connected to an anode terminal of the light emitting element, that applies a drive current to the light emitting element;
a capacitor element connected to a gate terminal of the driving transistor; and
a switching transistor connected between a first terminal of the capacitor element on the side of the gate terminal and a data line through which a desired program signal flows, wherein:
the driving transistor is an inorganic oxide thin film transistor whose OFF-operation threshold voltage is a negative voltage; and
a cathode terminal of the light emitting element and a second terminal of the capacitor element are connected to a common power source that supplies a predetermined common voltage.
A second display apparatus of the present invention is an apparatus, including:
an active matrix substrate on which the second pixel circuit of the present invention described above is disposed in a large number;
a data drive circuit that supplies the program signal; and
a common power source that supplies a predetermined voltage to the source terminal of the driving transistor and the second terminal of the capacitor element,
wherein a voltage value VB of the common voltage and a voltage value Vprg of the program signal are set such that the threshold voltage VTH, the voltage value VB of the common voltage, the voltage value Vprg of the program signal, a desired gate-source voltage VGS to be set for the driving transistor, and a forward voltage drop Vf across the light emitting element when the gate-source voltage of the driving transistor is VGS satisfy the relationships of Formulae (3) and (4) below.
VB≧−VTH (3)
Vprg=VGS−VB+Vf (4)
According to the first pixel circuit and display apparatus of the present invention, an inorganic oxide thin film transistor whose OFF-operation threshold voltage is a negative voltage is used as the driving transistor, and a source terminal of the driving transistor and a terminal of the capacitor element are connected to a common power source that supplies a predetermined common voltage. This enables a negative voltage to be applied between the gate and source of the driving transistor to cause the transistor to perform OFF operation even when the program signal is a positive voltage by supplying the common voltage while the capacitor element is being charged, which allows appropriate emission control in a low brightness region. Further, a conventional data drive circuit that outputs a program signal of positive voltage may be used without increasing power consumption.
According to the second pixel circuit and display apparatus of the present invention, an inorganic oxide thin film transistor whose OFF-operation threshold voltage is a negative voltage is used as the driving transistor, and a cathode terminal of the light emitting element connected to a source terminal of the driving transistor and a terminal of the capacitor element are connected to a common power source that supplies a predetermined common voltage. This enables a negative voltage to be applied between the gate and source of the driving transistor to cause the transistor to perform OFF operation even when the program signal is a positive voltage by supplying the common voltage while the capacitor element is being charged, which allows appropriate emission control in a low brightness region. Further, a conventional data drive circuit that outputs a program signal of positive voltage may be used without increasing power consumption.
Hereinafter, an organic EL display device to which a first embodiment of the pixel circuit and display apparatus of the present invention is applied will be described with reference to the accompanying drawings.
As shown in
Active matrix substrate 10 further includes multiple data lines 14, each for supplying the program signal outputted from data drive circuit 12 to each pixel circuit column, multiple scanning lines 15, each for supplying the scanning signal outputted from scan drive circuit 13 to each pixel circuit row, and multiple common power lines 17, each for supplying the common voltage outputted from common power source circuit 16 to each pixel row. Data lines 14 are provided orthogonal to scanning lines 15 and common power lines 17, forming a grid pattern. Each pixel circuit 11 is provided adjacent to the intersection of each data line 14 with each scanning line 15 and each common power line 17.
As shown in
Driving transistor 11b and switching transistor 11d are inorganic oxide thin film transistors whose OFF-operation threshold voltage is a negative voltage. The term “OFF-operation threshold voltage” as used herein refers to gate-source voltage VGS at which drain current ID start increasing rapidly, and the term “OFF-operation threshold voltage is a negative voltage” as used herein refers to that the transistor has, for example, a VGS-ID characteristic like that shown in
As shown in
Scan drive circuit 13 is a circuit that outputs ON-scan signal Vscan(on) and OFF-scan signal Vscan(off) for turning ON and OFF switching transistor 11d of pixel circuit 11 respectively.
Data drive circuit 12 is a circuit that outputs a program signal according to a display image to each data line 14.
Common power source circuit 16 is a circuit that supplies a common voltage to each common power line 17 with respect to each pixel circuit row.
An operation of the organic EL display apparatus of the present embodiment will now be described with reference to
First, a predetermined pixel circuit row is selected by scan drive circuit 13, and an ON-scan signal like that shown in
Then, as shown in
At the same time, a common voltage is supplied from common power circuit 16 only to common power line 17 connected to the pixel circuit row selected by scan drive circuit 13, and the potential of the common power line is raised from 0 v to VB, as shown in
Further, at the same time when the predetermined pixel circuit row is selected and ON-scan signal is outputted, a program signal according to desired brightness of display pixel of each pixel circuit 11 of the selected pixel circuit row is outputted from data drive circuit 12 to each data line 14, and the program signal outputted to each data line 14 is inputted to each pixel circuit 11 of the selected pixel circuit row.
Here, if the gate-source voltage to be set for driving transistor 11b is assumed to be VGS in order to cause organic EL element 11a of each pixel circuit 11 to emit light of desired brightness, voltage value Vprg of the program signal is set to Vprg=VGS+VB.
Consequently, charges according to voltage value Vprg of the program signal set in the manner as described above is stored in capacitor element 11c. Here, if the voltage held by capacitor element 11c is assumed to be Vcs, then Vcs=Vprg−VB=VGS.
After the charging of capacitor element 11c is completed in the manner as described above, an OFF-scan signal is outputted from scan drive circuit 13 to scanning line 15 to which the selected pixel row is connected.
Then, switching transistor 11d is turned OFF in response to the OFF-scan signal outputted from scan drive circuit 13, as shown in
At the same time when the selection of the predetermined pixel circuit row is released by the OFF-scan signal outputted from scan drive circuit 13, the potential of common power line 17 connected to the predetermined pixel circuit row is returned to 0 v from VB by common power source circuit 16. It is noted here that voltage Vcs of capacitor element 11c is maintained as it is.
Then voltage Vcs held by capacitor element 11c is applied to driving transistor 11b as gate-source voltage VGS, and a drive current flows through organic EL element 11a according to the applied voltage, whereby light is emitted from organic EL element 11a.
Here, voltage value Vprg of the program signal outputted from data drive circuit 12 is, Vprg>0 v, so that minimum value Vcsmin of voltage value Vcs settable to capacitor element 11c is, Vcsmin=−VB.
Accordingly, in order to cause organic EL element to stop the emission, that is, to cause driving transistor 11b to perform OFF operation, it is necessary to set voltage value VB of the common voltage outputted to common power line 17 from common power source circuit 16 as VB≧−VTH, where VTH is the OFF operation threshold voltage of driving transistor 11b.
As described above, by setting the potential of common power line 17 greater than or equal to −VTH while capacitance element 11c is being charged, a negative voltage may be set as gate-source voltage VGS of driving transistor 11b when Vprg>0 v and driving transistor 11b may perform OFF operation, as shown in
Thereafter, pixel circuit rows are sequentially selected by scan drive circuit 13, and charging and discharging of capacitance elements 11c are sequentially performed, whereby organic EL elements 11a sequentially emit light.
Where an inorganic oxide thin film transistor whose OFF-operation threshold voltage is a negative voltage is used also for switching transistor 11d as in pixel circuit 11 of the present embodiment, it is necessary to set the scanning signal supplied to switching transistor 11d to values shown below.
Vscan(off)≦VTH
Vscan(on)≧Vprmax+VTH
Thus, the scanning signal requires an amplitude ranging from a negative voltage to a positive voltage. Here, Vprgmax is the voltage value of the program signal corresponding to maximum brightness of organic EL element 11a.
As such, a scan drive circuit that outputs a positive voltage scanning signal may become usable by, for example, providing resistor elements R1 and R2 to each scanning line 15 connected to each pixel circuit row and a voltage source that supplies negative voltage Vee, as shown in
In the organic EL display apparatus according to the present embodiment, an inorganic oxide thin film transistor whose OFF-operation threshold voltage is a negative voltage is used as driving transistor 11b, so that power consumption is increased by the amount of common voltage VB supplied while capacitance elements 11c is being charged. But, in comparison with power consumption in the case where the ground wire of each pixel circuit is set to a voltage greater than 0 v, as shown in
Next, an organic EL display device to which a second embodiment of the pixel circuit and display apparatus of the present invention is applied will be described. The organic EL display device according to the second embodiment of the present invention differs from the organic EL display device according to the first embodiment of the present invention in the configuration of pixel circuit, although the general structure is identical to that of the organic EL display device of the first embodiment shown in
The pixel circuit of the second embodiment differs from the pixel circuit of the first embodiment in the position of the organic EL element. Whereas, in pixel circuit 11 according to the first embodiment, the cathode terminal of organic EL element 11a is connected to the drain terminal of driving transistor 11b, the anode terminal of organic EL element 11a is connected to the source terminal of driving transistor 11b in pixel circuit 21 according to the second embodiment, as shown in
In addition, as shown in
Other configurations of pixel circuit 21 are identical to those of pixel circuit 11 according to the first embodiment.
The operation of the organic EL display device according to the second embodiment is identical to that of the organic EL display device according to the first embodiment. But, it is necessary to set voltage value Vprg of the program signal outputted from data drive circuit 12 so as to satisfy the formula below.
Vprg=VGS−VB+Vf
where, Vf is the forward voltage drop across organic EL element 11a when the gate-source voltage of driving transistor 11b is VGS.
Each of the embodiments of the present invention described above is an embodiment in which the display apparatus of the present invention is applied to an organic EL display device. But, as for the light emitting element, it is not limited to an organic EL element and, for example, an inorganic EL element or the like may also be used.
The display apparatus of the present invention has many applications. For example, it is applicable to handheld terminals (electronic notebooks, mobile computers, cell phones, and the like), video cameras, digital cameras, personal computers, TV sets, and the like.
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